The present embodiments relate to wireless communication systems and, more particularly, to operation of a Coordinated Multi-Point (CoMP) communication system in which a user equipment (UE) simultaneously communicates with plural base stations (eNB).
With Orthogonal Frequency Division Multiplexing (OFDM), multiple symbols are transmitted on multiple carriers that are spaced apart to provide orthogonality. An OFDM modulator typically takes data symbols into a serial-to-parallel converter, and the output of the serial-to-parallel converter is frequency domain data symbols. The frequency domain tones at either edge of the band may be set to zero and are called guard tones. These guard tones allow the OFDM signal to fit into an appropriate spectral mask. Some of the frequency domain tones are set to values which will be known at the receiver. Among these are cell-specific reference signals (CRS), channel state information reference signals (CSI-RS), and demodulation reference signals (DMRS). These reference signals are useful for channel measurement at the receiver. Cell-specific reference signals as well as channel state information reference signals are not precoded and are generated by a pseudo-random sequence generator as a function of the physical cell ID. In Releases 8 through 10 of the Long Term Evolution (LTE) of the Universal Mobile Telecommunications System (UMTS), which was designed for conventional point-to-point communication, the cell ID is not explicitly signaled by the base station (called eNB) but is implicitly derived by the UE as a function of the primary synchronization signal (PSS) and secondary synchronization signal (SSS). To connect to a wireless network, the UE performs a downlink cell search to synchronize to the best cell. A cell search is performed by detecting the PSS and SSS of each available cell and comparing their respective signal quality, for example, in terms of reference signal received power (RSRP). After the cell search is performed, the UE establishes connection with the best cell by deriving relevant system information for that cell. Similarly, for LTE Release 11 the UE performs an initial cell search to connect to the best cell. To enable multi-point CoMP operation, the connected cell then configures the UE by higher-layer signaling with a virtual cell ID for each CSI-RS resource associated with each respective base station involved in the multi-point CoMP operation. The UE generates the pseudo-random sequence for each CSI-RS resource as a function of the virtual cell ID.
Conventional cellular communication systems operate in a point-to-point single-cell transmission fashion where a user terminal or equipment (UE) is uniquely connected to and served by a single cellular base station (eNB or eNodeB) at a given time. An example of such a system is Release 8 of the 3GPP Long-Term Evolution. Advanced cellular systems are intended to further improve the data rate and performance by adopting multi-point-to-point or coordinated multi-point (CoMP) communication where multiple base stations can cooperatively design the downlink transmission to serve a UE at the same time. An example of such a system is the 3GPP LTE-Advanced system. This greatly improves received signal strength at the UE by transmitting the same signal to each UE from different base stations. This is particularly beneficial for cell edge UEs that observe strong interference from neighboring base stations.
FIG. 1 shows an exemplary wireless telecommunications network 100. The illustrative telecommunications network includes base stations 101, 102, and 103, though in operation, a telecommunications network necessarily includes many more base stations. Each of base stations 101, 102, and 103 (eNB) is operable over corresponding coverage areas 104, 105, and 106. Each base stations coverage area is further divided into cells. In the illustrated network, each base station's coverage area is divided into three cells. A handset or other user equipment (UE) 109 is shown in cell A 108. Cell A 108 is within coverage area 104 of base station 101. Base station 101 transmits to and receives transmissions from UE 109. As UE 109 moves out of Cell A 108 into Cell B 107, UE 109 may be handed over to base station 102. Because UE 109 is synchronized with base station 101, UE 109 can employ non-synchronized random access for a handover to base station 102. UE 109 also employs non-synchronous random access to request allocation of uplink 111 time or frequency or code resources. If UE 109 has data ready for transmission, which may be user data, a measurements report, or a tracking area update, UE 109 can transmit a random access signal on uplink 111. The random access signal notifies base station 101 that UE 109 requires uplink resources to transmit the UE's data. Base station 101 responds by transmitting to UE 109 via downlink 110 a message containing the parameters of the resources allocated for the UE 109 uplink transmission along with possible timing error correction. After receiving the resource allocation and a possible timing advance message transmitted on downlink 110 by base station 101, UE 109 optionally adjusts its transmit timing and transmits the data on uplink 111 employing the allotted resources during the prescribed time interval. Base station 101 configures UE 109 for periodic uplink sounding reference signal (SRS) transmission. Base station 101 estimates uplink channel quality indicator (CQI) from the SRS transmission.
Downlink transmission in Long Term Evolution (LTE) is organized in subframes. Referring now to FIG. 2, there is a diagram of a downlink subframe in LTE. Each subframe 201 is of 1 ms time duration. Each subframe comprises twelve OFDM symbols with Extended Cyclic Prefix (CP) or fourteen OFDM symbols with Normal Cyclic Prefix (CP). The system bandwidth 215 consists of a plurality of L Physical Resource Blocks (PRB), where each PRB is composed of twelve OFDM tones called sub-carriers. The PRB is the smallest time-frequency resource allocation unit in LTE, where data transmission to a user is scheduled on one or multiple PRBs. Different PRBs in one subframe 201 are allocated for data transmission to different users. Furthermore, the set of PRBs on which a user receives downlink data transmission may change from one subframe to another.
In addition to downlink data, a base station also needs to transmit control information to mobile users. This includes both common control information as well as user-specific control information. Common control information is transmitted to all users in the cell to maintain users' connection to the network, page users in idle mode when a call comes in, schedule random access response, and indicate critical system information changes in the cell. In addition, user-specific control information is transmitted to each scheduled user, for example, to indicate the frequency resources on which the UE is expected to receive downlink data or transmit uplink data. In LTE, each subframe is divided into legacy control region 206 for downlink control information transmission and data region 207 for downlink data transmissions. The legacy control region 206 comprises OFDM symbols 1-3 when the system bandwidth is greater than 10 PRBs and OFDM symbols 2-4 otherwise. The exact size of the legacy control region is signaled on a Physical Downlink Control Format Indicator Channel (PCFICH). The data channel region 207 is located after the legacy control channel region 206 and is allotted for each Physical Resource Block (PRB). The legacy control channel region 206 is a region to which a Physical Downlink Control Channel (PDCCH) is mapped. The data channel region 207 is a region to which a Physical Downlink Shared Channel (PDSCH) is mapped and carries downlink data transmission to mobile users. Further, Enhanced Physical Downlink Control Channels EPDCCH Set 1 209 and EPDCCH Set 2 213 are frequency multiplexed with the data channel (PDSCH) 211 for transmission to a UE. That is, EPDCCH Set 1 209 and EPDCCH Set 2 213 are mapped to the data channel region 207 together with the PDSCH 211. The reason to locate the legacy control channel region at the beginning of the subframe is that a UE firstly receives a PDCCH allotted to the legacy control channel region 206 to recognize the presence of transmission of the PDSCH. Once the presence of transmission of the PDSCH is recognized, the UE may determine whether to perform a receiving operation of the PDSCH. If no PDCCH is transmitted to the UE, it is unnecessary to receive the PDSCH mapped to the data channel region 207. Accordingly, the UE may save power consumed in a receiving operation of the PDSCH. Meanwhile, the UE may receive a PDCCH located in the control channel region faster than the PDSCH 211 to reduce a scheduling delay. However, because the PDCCH is transmitted over the entire system bandwidth, interference control is impossible.
The legacy control channel region 206 may not be changed to a frequency multiplexing structure to maintain compatibility with an existing or legacy UE. However, if the eNodeB does not allot a corresponding region of the data channel region 207 to a UE of a previous LTE version, the UE of a previous LTE version does not receive a resource mapped to a corresponding data channel region 207. Accordingly, the eNodeB may transmit an EPDCCH for a UE of a new LTE version in a data channel region 207 that is not allotted to the UE. In other words, an EPDCCH being a control channel for a UE of a new LTE version has a structure multiplexed with the PDSCH.
While the preceding approaches provide steady improvements in wireless communications, the present inventors recognize that still further improvements in transmission of EPDCCH from the eNB to the UE are possible. Accordingly, the preferred embodiments described below are directed toward this as well as improving upon the prior art.